Its not too long a main story, but I mostly played it at the weekends when I have the most time. But I think they do encourage you to play more of the side missions rather than the main story so that you get the complete ending.

I heard somewhere that if you get 100% on all the side missions you get an alternate ending. I really like the game, my favorite part so far was the Pyg side quests. Sooo creepy. Too bad they canceled Silent Hills, I could really go for a good horror game.

Yeah, once you apprehend 7 of the Most Wanted, you get part of the ending and your closing credits, etc. Then once you get 100% on all side quests and Riddler Trophies, you get the full ending, added to the first one you were shown. Its weird.